Actively cooled effusion cell for chemical vapor deposition

ABSTRACT

An actively cooled effuser for a vapor deposition reactor is placed in very close proximity to a substrate. The actively cooled effuser has combinations of gas directing plates, cooling plates and isolation plates attached together. Reactants and coolant are input into the stack of plates so formed. Selective heating of the substrate surface may occur through the use of heating lamps. Multiple units of the actively cooled effuser and heating lamps may be used in the reactor to form multiple layers on the substrate. The cooling plate has a cooling channel within a few thousandths of an inch of the output side of the stack. The presence of the cooling plates allows the effuser to be placed in very close proximity to the selectively heated substrate.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government for governmental purposes without the payment of anyroyalty thereon.

BACKGROUND OF THE INVENTION

The present invention relates to chemical vapor deposition, and, inparticular, relates to apparatus of applying the chemicals to thesubstrate in question.

Chemical vapor deposition (CVD) is one of the methods of growingepitaxial layers on a substrate material.

In one prior CVD apparatus, the gaseous reactants are introduced into aquartz tube reactor vessel in which a substrate is held by a substrateholder and a susceptor. The susceptor is heated so that the reactantspyrolyze and deposit out onto the heated substrate thereon. The CVDapparatus includes a vapor delivery apparatus having the desired gasinput into a series of stacked flat plates positioned about a slottedgas pipe or rod. The stacked plates surround the slotted gas rod andform a series of flat flow channels to deliver the gas. The type of gasinput can be changed by valving into the vapor delivery apparatus.Because of the distance between the heated substrate and the vapordelivery apparatus, spent gas may recirculate affecting thickness andcompositional uniformity so that graded layer transistions occur whenchanging gas types. This results in a poor or inoperative semiconductorstructure.

One method of eliminating spent gases is to inject inert gases to cleanthe reactor vessel but thermal degradation of prior deposited layers canoccur while the inert gas is flushing the reactor vessel. The thermaldegradation of exposed surfaces of epitaxial layers also results indegraded crystal structures.

Another problem of prior apparatus is the pre-mature mixing orpre-reaction of the reactants before they reach the substrate.

A further problem with prior CVD reactors is non-uniform crystal layergrowth due to variations in the reactant gases. Prior solutions to thisproblem have been a long entrance length and/or inserts that helpproduce non-turbulent flow.

Another problem with prior CVD reactors is that only a small portion ofthe reactants actually deposit on the substrate so that one must collectand dispose of the spent reactant gases which are typically toxic.

Other problems with prior apparatus involve heating the substrate forproper chemical bonding where the reactant gases react. Several methodshave been used including resistive, infrared, inductive or convectiveheating. When the whole substrate is heated, uniformity of heating hasbeen a problem.

These problems clearly affect the ability to produce high qualitysemiconductor components such as high electron mobility transistors andmulti-quantum well laser diodes since they require extremely sharpheterojunctions between dissimilar layers of semiconductor crystals.

When heterojunctions are not sufficiently abrupt, grading and otherdefects occur which decrease component performance, reliability andlife. Creating abrupt heterojunctions is a difficult requirement toconsistently fulfill because many sophisticated heterojunctionstructures require abruptly switching of material in ten to twentyangstrom thick layers.

SUMMARY OF THE INVENTION

The present invention comprises a CVD reactor having at least oneactively cooled effuser with appropriate piping and valving, etc., asubstrate transporting means, a selective heating means, and otherfeatures to be detailed.

The actively cooled effuser has a plurality of gas directing plates anda plurality of interleaved cooling plates. The gas directing plates havean output side with an orifice therein. The cooling plates have acooling side with a cooling channel in close proximity thereto. When theplates are interleaved the cooling side and the output side are adjacentand in approximately the same plane. Appropriate piping and valvingintroduces the reactants and the coolant into the actively cooledeffuser. Additionally, isolation plates may be placed between gasdirecting plates where a subsequent gas follows the first. As a directresult of the cooling plates, the substrate is transported within a fewmillimeters of the actively cooled effuser. The heating means may be aheat lamp selectively focused to heat the areas of the substrate wherethe reactants are directed rather than the whole substrate.

An additional feature is an electric field between the actively cooledeffuser and the substrate to produce a plasma therebetween.

It is therefore one object of the present invention to provide a CVDreactor having an actively cooled effuser wherein the substrate may bepositioned in close proximity thereto for deposition to prevent gasrecirculation.

Another object of the present invention is to provide a CVD reactor thatprevents pre-reaction of the gases.

Another object of the present invention is to provide a CVD reactorcontaining selective heating means to eliminate bulk heating of thesubstrate.

Another object of the present invention is to provide a CVD reactor ableto produce high quality semiconductor structures having thin layers andabrupt interfaces on multiple substrates in a single CVD reactor.

Another object of the present invention is to provide a CVD reactor thatutilizes substantially less reactants.

And another object of the present invention is to provide a CVD reactorconstructed out of metal rather than glass.

These and many other objects and advantages of the present inventionwill be readily apparent to one skilled in the pertinent art from thefollowing detailed description of a preferred embodiment of theinvention and the related drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a CVD reactor having a plurality of effusers withheating lamps.

FIG. 2 illustrates by view a single actively cooled effuser.

FIG. 3A illustrates by side view a single gas directing plate.

FIG. 3B illustrates by side view an isolation plate.

FIG. 3C illustrates a slotted feed rod.

FIG. 3D illustrates a gas directing plate with a center feed.

FIG. 4 illustrates by a cross section view a cooling plate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, a chemical vapor deposition (CVD) reactor 10 ispartially shown. The CVD reactor 10 includes a gas/coolant supply 12,valving means 14, input means 15, venting means 16, and transportingmeans 18 having at least one substrate 20 thereon. A reaction chamber 22being enclosed by a metal wall 24, shown partially, has therein twoactively cooled effusers 26 and 28 with heating means 30, 32 and 34 fordepositing on the substrate 20 a lay of niobium carbide followed by alayer of niobium nitride. Other types and numbers of layers are clearlypossible as well as the number of heating means and effusers. Thesubstrate 20 is moving to the right as is indicated by arrow 46. Thisembodiment is only considered to be illustrative of the apparatus andthe technique involved.

As the substrate 20 is transported to the left, heating means 34 being aheating lamp 36 in a housing 38 with an optional focusing means 40causes light energy 42 to fall upon the substrate 20 and heat it to arequired reaction temperature at a surface 44 of the substrate 20. Firstreactants 48 are emitted from the actively cooled effuser 28 onto theheated surface 50. Since the reactants are deposited only on thesurface, heating is only selectively required at this location and onlyto a depth of about 5 microns or so. The actively cooled effuser ispositioned within about a few millimeters of the surface 44 thusreducing recirculation of spent gases and pre-reaction of the gases.

The subsequent heating means 32 and the actively cooled effuser 26deposit a second layer 54 on a first layer 52.

The reactants supplied by the gas supply 12 are directed through thevalving means 14 to the appropriate actively cooled effuser. The ventingmeans 16 removes the spent reactants from the reaction chamber 22.

The transporting means 18 may be either a rotary table or a linearlymoving table with either single or multiple substrates thereon. Further,the transporting means 18 may alternatively move the actively cooledeffusers and heating means.

Referring to FIG. 2, a single actively cooled effuser 56 is shown.

The Effuser 56 has a coolant input 58 and a coolant output 60 feedinginto and out of a coolant manifold 62. The coolant manifold 62 isfurther connected to a number of cooling plates 64, FIG. 4, which areinterleaved with gas directing plates 66, FIG. 3A or 3D. If multiple gasdirecting plates 66 are grouped between coolant plates 64, isolationplates 68, FIG. 3B, may be used to separate these.

As to the gas directing plates 66 and the isolation plates 68 referenceis made to U.S. Pat. No. 4,736,705 which is incorporated by reference.

Referring back to FIG. 2, gas inputs 70 are connected to a pair ofslotted feed rods 72, such as shown in FIG. 3C, having a longitudinalslot 74 therein. As the gas flows into rod 72, it exits through the slot74 in accordance to the positioning of the gas directing plates 66.

Referring to FIG. 3A, the gas directing plate 66 shown has two gasinputs. The dotted lines indicate the structure when a third gas input76, FIG. 3D, is included. A third rod 72 is inserted into the hole 96.

In order to assemble plate stack 78, FIG. 2, two slotted feed rods 72are inserted into coupling blocks 80. The coupling blocks 80 areattached to the cooling plate 64. Nextly, the gas directing plate 66 isplaced over the rods 72 and next to the cooling plate 64. Then anothercooling plate 64 is placed on rods 72 and next to the gas directingplate 66.

Again referring to FIG. 3A, the gas directing plate "66 shown isconsidered a "left rod plate 82 since the rod 72 passes through a hole84 with a flow channel 86 fluidly connected thereto and flow channel 86is fluidly connected to a gas directing section 88. As the gas flowsinto rod 72 it exits through the slot 74 into the flow channel 86 andthen into the gas directing section 88 from there exiting through anoutput side 90 having an orifice 92 therein. The gas flowing in theright rod 72 through hole 94 is not able to enter gas directing section88 because there is no flow channel at the section.

In order for the gas to exit from the right rod, gas directing plate 66is flipped over when it is attached to the plate stack 78 to form a"right rod plate." Each gas directing plate 66 has either the coolingplate 64 and/or the isolation plate 68 on each side while in the platestack 78.

A third feed rod 72 may be placed in a third hole 96, FIG. 3D. In orderfor the gas to flow from the third rod 72 only, only the center flowchannel 98 can exist in that plate as shown.

As noted above each gas directing plate 66 has either the isolationplate 68 and/or the cooling plate 64 on both sides unless there is adesire to either pre-mix or pre-react the gases.

The cooling plate 64 is shown in detail in FIG. 4. The holes 84 and 94for the rods 72 pass directly through. A flow channel 100 connected tothe coolant manifold 62 passes within a few thousandths of an inch to acooling side 98. Even when the substrate surface 50 is within a distanceof about 0.010 to 0.1 inches, the output side 90 of gas directing plates66 should remain cool when the surface 50 is at about 800 degreesCentigrade. Reflecting material may be applied to the cooling side 98 toreduce the heating affect.

Another feature of the invention is a plasma producing means 102 shownin FIG. 2 wherein an DC or AC voltage is applied between plate stack 78and the substrate 20. The close proximity of the plate stack 78 to thesubstrate 20 allows for modest voltages to create the required electricfield to induce the plasma state.

Clearly, many modifications and variations of the present invention arepossible in light of the above teachings and it is therefore understood,that within the inventive scope of the inventive concept, the inventionmay be practiced otherwise than specifically claimed.

What is claimed is:
 1. A vapor deposition reactor, said vapor depositionreactor comprising:a reaction chamber, said reaction chamber havingwalls thereabout to confine reactants therein; means for inputtingreactants and coolant into said reaction chamber; means for ventingreactants from said reaction chamber; means for transporting at leastone substrate about said reaction chamber for the purpose of depositionof reactants on said at least one substrate; at least one effuser, saideffuser being in close proximity to said substrate for depositing saidreactants thereon in a laminar flow condition preventing recirculationof spent reactants, said effuser including means for cooling an outputside being in close proximity to said substrate, heat from saidsubstrate in close proximity to said effuser being removed from saidoutput side to prevent pre-action of reactants to be deposited; and atleast one means for heating said substrate selectively in an area toreceive deposition, said at least one means for heating being positionedbefore an effuser, the area of said substrate only being heatedimmediately adjacent to said effuser whereby heat is only applied to thearea, a remaining area of said substrate not being heated by said meansfor heating.
 2. A vapor deposition reactor as defined in claim 1 whereinsaid at least one actively cooled effuser is within 10 millimeters ofthe surface of the substrate.
 3. A vapor deposition reactor as definedin claim 1 further including means to cause said reactants to become aplasma between said at least one actively cooled effuser and said atleast one substrate.
 4. A vapor deposition reactor as defined in claim 1wherein said cooling plates and said gas directing plates arealternately attached to form a plate stack, said plate stack attached toat least one source of reactants and a source of coolant, said platestack having the output side.
 5. A vapor deposition reactor as definedin claim 4 wherein said output side of said plate stack is coated with aheat reflecting material.
 6. A vapor deposition reactor as defined inclaim 1 wherein said means for heating is a heat lamp adjacent to atleast one actively cooled effuser to selectively heat the area of thesubstrate upon which deposition occurs.
 7. A vapor deposition reactor asdefined in claim 6 wherein said heat lamp includes means to focus thelight energy on said at least on substrate.